2 TRIS(IMINO)TIN(II) AND TRIS(IMINO)LEAD(II) CAGE COMPLEXES - SYNTHESES AND STRUCTURES OF E[MU-N=C((T)BU)PH]3LI-CENTER-DOT-THF (E=SN, PB)

The reactions of Cp2Pb and Cp2Sn with LiN=C((t)Bu)Ph (1:1; 2 or 3 equiv) in THF produce the isostructural complexes Pb[mu-N=C((t)Bu)Ph]3Li.THF (1) and Sn[mu-N=C((t)Bu)Ph]3Li.THF (2), respectively. Complex 2 can also be prepared by the facile ligand-exchange reaction of 1 and Cp2Sn in THF. The crystal structures of both complexes have been obtained at 153 K. Crystal data: 1, orthorhombic, space group P2(1)2(1)2, a = 16.912(2) angstrom, b = 19.214(2) angstrom, c = 11.212(3) angstrom, Z = 4; 2, monoclinic, space group P2(1)/n, a = 10.679(2) angstrom, b = 33.036(7) angstrom, c = 10.693(2) angstrom, beta = 106.84(3)degrees, Z = 4. Both complexes have monomeric trigonal bipyramidal EN3Li cage structures (E = Pb, Sn) in which there are sharp (ca. 80-degrees) angles at the bridging imino-N centers and close intermolecular E...Li contacts which are slightly greater than the sum of the covalent radii of the metals [2.85(3) in 1 and 2.776(4) angstrom in 2]. Model semiempirical (MNDO) calculations suggest that there is no significant E-Li bonding in either complex and that the observed core geometries mainly reflect the optimal bridging angle of the imino ligands. Monomer molecules of 1 exhibit extreme angular distortions within the PbN3Li core, resulting from two such molecules pairing up in the lattice. Their phenyl groups intermesh, and long-range (ca. 3.26 angstrom) (ortho)C-H...Pb contacts are established. In contrast, molecules of 2 have a largely symmetrical core and do not associate in the manner of those of 1 in the solid state. This unexpected structural difference between 1 and 2 may suggest that the (ortho)C-H...Pb contacts observed in 1 are more than simply van der Waals interactions.